Microwave spectroscopy and quantum chemical investigation of nine low energy conformers of the 15-crown-5 ether

Abstract

Crown ether macrocyles constitute a central class of selective substrates in Supramolecular Chemistry. The backbone of crown ethers is particularly flexible which makes the spectroscopic and computational investigations challenging. In this work, we present a systematic investigation of the low energy conformational landscape of the benchmark macrocycle 15-crown-5 ether (15c5) under isolated conditions. Molecular beam Fourier transform microwave spectroscopy is employed to measure a broad ensemble of rotational transitions within the range of 4-11 GHz. The recorded spectra allow us to determine the rotational constants of eight individual 15c5 rotamers which are identified among the nine lowest energy conformers predicted by ab initio MP2 computations. The study illustrates well the variety of conformers of prolate and oblate character that the 15c5 molecule displays within a narrow range of energies. Implications for the supramolecular behavior of 15c5 are discussed.

Medium-sized rings: conformational preferences in cyclooctanone driven by transannular repulsive interactions

“…Structural investigations of larger cyclic ketones will yield information on the balance of intramolecular forces in more flexible and complex rings and help elucidate possible trends as the size of the ring increases. These results can be compared with those from recent rotational studies of large ring crown ethers [51][52][53] , where attractive CH…”

Water induces the same crown shapes as Li+ or Na+ in 15-crown-5 ether: a broadband rotational study

“…16,17 The coupling between skeletal modes make the interconversion between conformers through reasonably low barriers possible. 13 In fact, the potential energy surfaces (PES) of CEs contain multiple energy minima with low interconversion barriers, [16][17][18] resulting in a high flexibility and giving rise to a rich conformational landscape. This flexibility enables transformation mechanisms to maximize host-guest interactions in the formation of complexes.…”

“…Nevertheless, the structure and properties of molecules and adducts of increasing size investigated using CP-FTMW is an active field of research. 13,[20][21][22][23] Only a limited number of gas-phase studies of crown ethers 7,[16][17][18] or their complexes with water [7][8][9][10][11][12][13] or other solvents 14 can be found in the literature. The first gas phase works on CE-water complexes were done by Ebata and co-workers 7-11 on benzo-18-crown ether-(H 2 O) n and dibenzo-18-crown ether-(H 2 O) n and Shubert et al 12 on benzo-15-crown ether-(H 2 O) n and 4'-aminobenzo-15-crown ether-(H 2 O) n , all of them bearing a chromophore group that enabled the use of laser spectroscopy techniques in supersonic jets.…”

“…MW spectroscopy provides access to molecules without chromophore groups, thus widening the range of molecules accessible to study. The only two systems studied by MW spectroscopy are 15-crown-5 ether (15C5) 16,17 and 12-crown-4 ether-(H 2 O) n . 13 In the previous microwave work on the 15C5 monomer 16,17 performed with a narrowband molecular-beam FTMW spectrometer (MB-FTMW) based on a Fabry-Pérot type resonator design, the spectra of eight different conformers of 15C5 crown ether were observed following the guide of formerly reported ab initio calculations.…”

Water-Induced Structural Changes in Crown Ethers from Broadband Rotational Spectroscopy

“…Up to now, only a limited number of gas-phase studies of CEs 6,7,9 and their complexes with water or other solvents can be found in the literature. [10][11][12][13] Pioneering work was performed on benzo-CE compounds, bearing a chromophore group, using IR-UV laser spectroscopy techniques.…”